The invention relates to magnetic resonance imaging (MRI), and in particular to high speed echo-planer imaging (EPI) techniques.
EPI is a commonly used MRI technique for high speed acquisition of NMR data, in which scan times are generally about 100 msec. For the simplicity of discussion, the Z-axis is used as the slice selection direction, the X-axis is used as the phase-encoding direction and the Y-axis is used as the readout direction. Other orientations may be applied when using the invention described herein.
As shown in FIGS. 1 and 2, in a conventional EPI pulse sequence, a 90° radio frequency (RF) excitation pulse 10 with a slice selective magnetic field gradient (Gslice) 12 is applied along an axis perpendicular to the imaging plane, e.g., (Gz), to excite the nuclei in the imaging plane of the body being imaged. A phase encoding gradient (Gphase) 14 and 24 is applied, along an axis (Gx) parallel to the imaging plane, after the excitation pulse to spatially encode the nuclei. Similarly, a phase shift gradient (Gread) 16 is applied, along an axis (Gy) parallel to the imaging plane and orthogonal to the phase encoding gradient, to center the subsequent scanning of the k-space (raw data space). A 180° RF rephasing pulse 18 is applied to generate a spin echo (SE) response (ADC) 20 from the excited nuclei. A slice specific gradient 19 may also be applied in conjunction with the 180° RF pulse.
During a signal sampling period, an alternating readout magnetic field gradient (Gread) 22 is applied to scan k-space and acquire SE signal samples 20 from the excited nuclei. In combination with the readout gradient, a continuous phase encoding gradient (Gphase) 24 may be applied to cause the scanning to move along the phase encoding (Gx) direction, as is shown in FIG. 3. The scan trajectory 26 forms a zig-zag pattern through k-space due to the alternating readout gradient 22 and the continuous phase encoding gradient 24. Alternatively, the phase encoding gradient may be applied as blip pulses 28 aligned with the reversal of the readout gradient to shift the scan trajectory 30 after each pass through a row of k-space, as is shown in FIG. 4.
As is shown in FIGS. 3 and 4, data is generally sampled during an EPI sequence in a raster scan trajectory through k-space, where individual scan lines corresponding to the readout gradient are sequentially sampled. After each scan line 32 is sampled, the k-space trajectory is shifted along the phase gradient direction to a next scan line 34. The reversal of the readout gradient 22 causes the k-space trajectory to reverse along the readout gradient. By reversing the trajectory, the scan through k-space can proceed back and forth along the read-out gradient on a line by line sequence.
The phase encoding gradients 24, 28 are perpendicular to the line by line trajectory of the data acquisition trajectory. Data along a line parallel to the phase encoding gradient is acquired slowly during the course of an entire scan of k-space. In contrast, data acquired along each line parallel to the readout gradient is acquired quickly as the scanning trajectory passes through one line of the scanning trajectory. Accordingly, data in the phase encoding gradient direction is acquired at a much slower rate than is data collected along the readout gradient direction.
The NMR signal samples acquired during the readout gradient may be transformed from the k-space domain to a spatial domain using conventional mathematical techniques, such as a Fourier transform. Data in the spatial domain is used to generate a NMR image of a cross-section of the body corresponding to the slice selected for imaging.
Images generated using an EPI sequence are susceptible to distortion and artifacts caused by magnetic field inhomogeneities and other abnormalities of the MRI system. With respect to high speed images generated using EPI sequences, the image distortions are particularly acute along the phase encoding direction because of the relatively slow data sampling rate along that direction.
Induced magnetic field distortions are a source of image distortions. An induced field distortion arises when a magnetic field is induced by a switched gradient magnetic field in an MR imaging sequence, an EPI sequence. The induced field is a cross-field when it is orthogonal to the inducing gradient field. Induced magnetic cross-field distortions may result from eddy-currents (EC) and Maxwell electromagnetic fields in the MRI system. During an EPI sequence, an induced cross-field may arise along the phase-encoding direction due to the rapidly switched readout gradient during the data sampling period.
In view of the relatively slow sampling rate along the phase-encoding direction (Gphase), the gradient induced cross-field due to a switched readout gradient (Gread) may result in substantial image distortions along the phase-encoding direction. The image distortion may be particularly acute in images generated from an EPI sequence where the readout gradient is repeatedly reversed during the data sampling period. There is a long-felt need for techniques to compensate for induced magnetic cross-fields that create image distortions, especially for distortions resulting from EPI sequences during which induced cross-fields are generated by the readout gradient.